I personally think K2 did as well as it could have done.
2m for me was realistic and due to the date of the release and the PS3 install base at the time. I don't think sony or GG could have expected much more.
I would say 3m K2 sales LTD would be far better than any sensible person would have hoped.
The great thing is that now it has a huge fanbase (according to GG) and an expensive engine all ready for the 3rd installment.
I and many others who now play K2 never even heard of the original.
It is not like Killzone was a PS flagship title.
I think U2 will get close to MGS sales LTD as the PS3 install base grows due to it's unofficial reputation as the best looking game on a console.